A medical image is obtained from a medical image capturing apparatus, and is displayed on a display apparatus. A doctor observes and diagnoses the medical image displayed on the display apparatus. In some cases, the medical image to be observed includes a plurality of luminal organs such as blood vessels, bronchial tubes, and intestines in a complicated form, depending on a portion from which the medical image is captured. A method of observing such luminal organs is disclosed in, for example, Non-Patent Document 1. In the Non-Patent Document 1, a display displays a medical tomographic image captured by means of a medial tomographic image capturing apparatus. By reference to the displayed medical tomographic image, an operator sets, by use of a mouse, a starting point of a region for extracting a luminal organ region to be observed. Through use of a predetermined extraction algorithm, a CPU extracts the luminal organ region from the set starting point, and detects a branching portion of the extracted luminal organ region. The CPU then extracts, through use of the predetermined extraction algorithm, luminal organs extending from the detected branching portion.    Non-Patent Document 1: A. Etienne, R. M. Botnar, A. M. C. Muiswinkel, et al.: Soap-Bubble visualization and quantitative analysis of 3D coronary magnetic resonance angiograms, Magn Reson Med Vol. 48, pp. 658-666 (2002).